Enumerators cannot open rejected interviews: "interview case is invalid"

Hello everybody,

Our enumerators are unable to open rejected interviews in the browser or interviewer app.
In the interviewer app the error message is “Failed to open interview: Interview case is invalid and cannot be opened” and in the browser “Error: changeSection: Cannot send data if the connection is not in the ‘Connected’ State”. I copy them below.
I found an earlier post on the same error message in the browser but in difference to that our supervisors and headquarters users are not reporting any problems.
My colleague from IT tried to follow the advice given in relation to that earlier post but could not fix the problem yet. He tried to enable and update the WebSocket but this did not help.

We use Internet Information Services (IIS) from Windows Server 2019 Datacenter to host Survey Solutions.

I would be very thankful for some quick help as the interviewers are already in the field.

Best regards,

Lutz

browser screenshot (2)